1. Field of the Invention
The invention relates to a platinum-free chelate catalyst material for the selective reduction of oxygen with at least one unsupported transition metal, a nitrogen and a carbon component structured as a porous conductive carbon matrix into which the unsupported transition metal and a transition metal chelate coordinated by the nitrogen are respectively bonded as electron donor and catalytic center, and to a method of its production.
2. The Prior Art
A major field of application of catalysts are electrochemical cells and, in this context, emission-free fuel cells in particular, for generating electricity efficiently and in an environmentally friendly manner by converting the chemical energy of a fuel oxidation reaction into electrical energy without prior heat generation. The fuel cell is particularly efficient if hydrogen is converted to electrical energy. From among the many types of fuel cells, the polymer electrolyte membrane (PEM) which is suitable in an excellent manner for operation with hydrogen/oxygen or hydrogen/air is to be specially mentioned. In this connection, the low energy density of hydrogen is a problem, however, which is the reason for the increasing use of methyl alcohol as a fuel. On the one hand, methyl alcohol can be catalytically converted into hydrogen and carbon dioxide in a vehicle (indirect methyl alcohol fuel cell) or, on the other hand, it can be directly oxidized at the anode. In this context, the one which can be realized in the technically simplest way is the direct methanol/air fuel cell (DMFC). For that reason, it may be considered to be a highly promising electrochemical source of energy for small appliances and electric motors. Hitherto, mixed platinum/ruthenium sponges or so-called “carrier catalysts” have primarily been used as catalyst materials for the anode in DMFC's, in which minute metal particles are precipitated on a conductive carrier material such as carbon black or graphite. Pure platinum or supported platinum are used as catalyst for the cathode. However, pure platinum does not act selectively, and problems arise, therefore, if methane is used as the fuel. While the cathode and the anode are separated from each other by a proton-conductive membrane as a connector between them, it is pervious to methane which may reach the cathode where it will be oxidized as well. In this manner, the cathode which is to reduce the air oxygen, depolarizes and reduced conduction will occur.
The present invention resides in the field of platinum-free catalyst materials which reduce selectively and which are thus resistant to alkanoles, and, in this context, within the group of chelate catalyst materials. A chelate is a catalytically very active higher order complex compound in which a central metal ion, forming several compounds, is surrounded in the manner of a ring by one or more molecules or ions. Different platinum-free, methanol-resistant chelate catalyst materials in supported or unsupported form have already been described in scientific literature. Yet none of the known types of catalyst material hereinafter to be described have been technically used as their catalytic activity and stability cannot be judged to be sufficient. The presence of highly conductive carbon of a large specific surface is essential for technical applications. Not only does the high-temperature reaction of the chelates result in improved activity but it also increases the stability of the catalyst material. In this connection it is necessary to distinguish between direct feeding of conductive carbon, such as, for instance, carbon black, and an in-situ-production of the carbon matrix by the polymerization of suitable oregano-metallic chelates to which the invention relates also.
The article [I] “direct methanol—air fuel cells for road transportation” (B. D. McNicol et al., Journal of Power Sources 83 (1999) pp. 15-31, describes catalyst materials with non-noble metals for use in DMFC (Chapter 4.5.2). Alternative preparations of organo-metallic chelates such as iron or cobalt porphyries and phtalo cyanines as well as tetraazaannulene are being described. In this connection, in a metal tetra phenyl porphyrene as the active chelate, a metal ion is surrounded by four nitrogen atoms (MeN4) each of which is bonded to a monopyrrol ring. The catalytic activity of these compounds for oxygen reduction has also been known for some time. Different transition metals used in the chelates lead to different results. Whereas the use of cobalt leads to a significantly increased activity, iron results in a marked increase in stability. Even if some of the reports relate to a very good catalytic activity, these materials nevertheless do not at present display sufficient stability to be useful in fuel cells.
The prior publication [II] by Contamin et al. reports upon the preparation of a cobalt-containing electrocatalyst by pyrolysis of cobalt tetraazaannulene in the presence of active charcoal soot (see O. Contamin, C. Debiemme-Chouvy, M. Savy and G. Scarbeck: “Oxygen electroreduction catalysis: Effect of Sulfur Addition on Cobalt Tetraazaannulene Precursors”, Electrochimica Acta 45 (1999), pp. 721-7291. When adding thio urea to the starter preparation, the authors observed a significant increase in the activity of the catalyst. The active center consists of two oppositely positioned cobalt atoms bonded to the carbon matrix by C—S-bridges.
JP 59138066 describes the production of a catalyst material by mixing transition metal compounds with cobalt, copper, nickel, molybdenum, and/or tin with iron, urea and, for instance, a pyromellitinic acid anhydride followed by a temperature treatment in the presence of a conductive carbon substrate. It results in a metal-phtalo-cyanine-polymer with an integration of the used different transition metals which are bonded as cores into the metal chelates. The material is being proposed for use in alkaline fuel cells. As regards the parallel use of several different transition metals, a scientific paper is yet to be mentioned, which reports on the catalytic activity of unsupported mixtures of cobalt tetraphenylporphyrine (COTPP) and iron tetraphenylporphyrene (FeTPP). In accordance with the publication [III] by R. Jiang and D. Chu (“Remarkably Active Catalysts for the Electroreduction of O2 to H2O for Use in an Acidic Electrolyte Containing Concentrated Methanol”, Journal of the Electrochemical Society 147 (12), pp. 4605-4609 (2000)), the binary mixture of CoTPP and FeTPP treated at 600° C. under argon displayed an increased catalytic activity relative to pure temperature-treated substances. The structure of the material is, however, relatively compact and has no significant porosity.
U.S. Pat. No. 6,245,707 describes methanol-tolerant electrocatalysts for the oxygen reduction on the basis of nitrogen-chelates with at least two different transition metals (e.g. metal tetraphenylporphyrene), which in the presence of a carbon support are converted by thermal treatment to an active cathode catalyst for use in low temperature fuel cells.
Mixing of a ferrous salt (iron acetate) with perylene tetracarboxyanhydride (PTCDA) followed by temperature treatment in the presence of ammonia (NH3) gas for producing a chelate catalyst material is known from the essay [IV] “Oxygen reduction catalysts for polymer electrolyte fuel cells from the pyrolysis of Fell acetate adsorbed on 3,4,9,10 perylene tetracarboxylic dianhydride” by G. Faubert et al. (Electrochimica Acta 44 (1999), pp. 2589-2603). The PTCDA produces a porous conductive carbon matrix, and NH3 introduces nitrogen. In the introduction of this essay particular attention is directed to the fact that for producing a stable non-noble metal-based catalyst material a transition metal such as Fe or Co derived from a salt, as well as a nitrogen and a carbon source, are required. This may be realized in situ by polymerization of the carbon source.
The prior art from which the invention is proceeding is described in the essay [V] “O2 Reduction in PEM Fuel Cells: Activity and Active Site Structural Information for Catalysts Obtained by Pyrolysis at High Temperature of Fe Precursors” by M. Lefévre et al. (J. Phys. Chem. B (2000)). In this context, Fell acetate as precursor compound is mixed with PTCDA as organic compound in the presence of NH3 as nitrogen precursor compound and is pyrolyzed at a high temperature in excess of 800° C. The polymerization of the metal and nitrogen-free PTCDA results in situ in a porous conductive carbon matrix into which individual iron atoms are adsorptively bonded as electron donors and as iron chelate coordinated by four nitrogen atoms. The essay reveals that the catalyst activity of the chelate catalyst material may be affected by way of the iron content and the temperature of the pyrolysis. However, this is insufficient for any commercial application which is based not least on the relatively low attained porosity. Furthermore, no adequate stability can be attained. Moreover, in the synthesis, a matrix former as well as a nitrogen donor separated therefrom, must be used in addition to the transition metal.